Secure beamforming for cognitive cyber-physical systems based on cognitive radio with wireless energy harvesting

Cognitive cyber-physical systems (CPS) have been extensively applied in many fields from intellectual traffic control system to autonomous robot for face-detection and face-recognition. In wireless-powered cognitive radio networks (WP-CRN), it is necessary to provide secure and effective real-time feedback for CPS users. In this paper, we consider secure transmission issues in WP-CRN for CPS users, where a primary system works in conjunction with a wireless-powered secondary system in the presence of multiple potential eavesdroppers. Furthermore, a novel spectrum sharing scheme is proposed, which multiplexes the secondary transmissions during both the primary and cognitive transmissions. We aim to maximize the secondary data rate while guaranteeing the secure rate requirement of the primary system by jointly optimizing the energy harvesting duration and the beamforming vectors of the primary and secondary systems. In order to solve the non-convex optimization problem, the semi-definite relaxation (SDR) with Charnes-Cooper transformation is adopted to transfer the non-convex problem into convex optimization problem, which can be easily solved by CVX tools. Moreover, we can prove that the SDR is tight since there always exists a rank-one optimal solution. Simulation results illustrate that the proposed scheme can significantly improve the secondary transmission rate while can achieve higher security rates of the primary system compared with zero-forcing (ZF) beamforming scheme.